Field of the Invention
The invention pertains to the field of variable cam timing. More particularly, the invention pertains to a variable camshaft timing mechanism with at least one lock pin engaged by oil pressure.
Description of Related Art
Internal combustion engines have employed various mechanisms to vary the relative timing between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more “vane phasers” on the engine camshaft (or camshafts, in a multiple-camshaft engine). As shown in the figures, vane phasers have a rotor 105 with one or more vanes 104, mounted to the end of the camshaft, surrounded by a housing assembly 100 with the vane chambers into which the vanes fit. It is possible to have the vanes 104 mounted to the housing assembly 100, and the chambers in the rotor assembly 105, as well. The housing's outer circumference 101 forms the sprocket, pulley or gear accepting drive force through a chain, belt, or gears, usually from the crankshaft, or possibly from another camshaft in a multiple-cam engine.
Apart from the camshaft torque actuated (CTA) variable camshaft timing (VCT) systems, the majority of hydraulic VCT systems operate under two principles, oil pressure actuation (OPA) or torsional assist (TA). In the oil pressure actuated VCT systems, an oil control valve (OCV) directs engine oil pressure to one working chamber in the VCT phaser while simultaneously venting the opposing working chamber defined by the housing assembly, the rotor assembly, and the vane. This creates a pressure differential across one or more of the vanes to hydraulically push the VCT phaser in one direction or the other. Neutralizing or moving the valve to a null position puts equal pressure on opposite sides of the vane and holds the phaser in any intermediate position. If the phaser is moving in a direction such that valves will open or close sooner, the phaser is said to be advancing and if the phaser is moving in a direction such that valves will open or close later, the phaser is said to be retarding.
The torsional assist (TA) systems operates under a similar principle with the exception that it has one or more check valves to prevent the VCT phaser from moving in a direction opposite than being commanded, should it incur an opposing force such as a torque impulse caused by cam operation.
The auto industry has determined there are multiple strategies that can be used with an intake camshaft phasing mechanism. For example, a camshaft phaser locked at some intermediate start position is best for cold engine start emissions. An intake camshaft phaser commanded to full retard position is best for improved fuel economy during engine operation.
The problem with OPA or TA systems in executing the strategies discussed above is that the oil control valve defaults to a position that exhausts all the oil from either the advance or retard working chambers and fills the opposing chamber. In this mode, the phaser defaults to moving in one direction to an extreme stop where the lock pin engages. A bias spring may be used to preferentially guide the phaser to a desired position. The OPA or TA systems are unable to direct the VCT phaser to any other position during the engine start cycle when the engine is not developing any oil pressure. This limits the phaser to being able to move in one direction only in the engine shut down mode. In the past this was acceptable because at engine shut down and during engine start the VCT phaser would be commanded to lock at one of the extreme travel limits (either full advance or full retard).
Furthermore, by reducing the idling time of an internal combustion engine in a vehicle, the fuel efficiency is increased and emissions are reduced. Therefore, vehicles can use a “stop-start mode” which automatically stops and automatically restarts the internal combustion engine to reduce the amount of time the engine spends idling when the vehicle is stopped, for example at a stop light or in traffic. This stopping of the engine is different than a “key-off” position or manual stop via deactivation of the ignition switch in which the user of the vehicle shuts the engine down or puts the car in park and shuts the vehicle off. In “stop-start mode”, the engine stops as the vehicle is stopped, then automatically restarts in a manner that is nearly undetectable to the user of the vehicle. During “stop-start”, it has been determined that the full retard phaser position reduces the energy required to start the engine and the full retard phase position reduces the engine Noise Vibration and Harshness (NVH) during a hot engine restart. Other strategies may be developed that require a different lock position than described.
The problem with an intake camshaft phaser design that has an extended range of authority and the ability to lock at the full retard stop is that if the engine is shut down with the intake camshaft phaser locked at or near the retard stop and the engine is allowed to cool down, then the engine may not be able to accomplish a successful cold start with the phaser locked near the retard stop. Therefore, it is desirable for the phaser to be unlocked and repositioned to the mid lock position during engine cranking A typical hydraulic operated camshaft phaser uses a spring force to engage the lock pin and engine oil pressure to release the lock pin. However, during engine cranking there may not be sufficient engine oil pressure to release the lock pin.